Method for the preparation of a multilayer coating and aqueous coating material suitable for this

Abstract

Described are a method for the preparation of a multilayer coating and a coating material from an aqueous dispersion of a polymer with a particle diameter of the polymeric phase of 10 nm to 500 nm. The polymer has a number average molecular weight of 10,000 to 500,000 and an acid number of 12 to 40. The dispersion was obtained by polymerization without addition of emulsifier and in the presence of water insoluble initiators of a) 0.65 to 9 parts by weight of carboxyl group-free unsaturated monomers which, aside from the unsaturated bond, do not contain any other groups, which are reactive under the polymerization and curing conditions, together with 0 to 65% by weight, based on the total monomers of copolymerizable, hydroxyl group-containing monomers and/or 0 to 7% by weight, based on the total monomers, of ethylenically unsaturated monomers, in b) 1 part by weight, based on the resin portion, of an aqueous dispersion of a urea group-containing polyurethane, which was produced by chain extension of an NCO group-containing prepolymer based on polyester with an NCO group content of 1.0 to 10% with at least two urethane groups per molecule, with carboxyl groups corresponding to an acid number of 20 to 50 and a number average molecular weight of 600 to 6,000, with a polyamine with primary and/or secondary amino groups and/or with hydrazine in an aqueous medium without addition of emulsifier.

Description

The invention relates to a method for the preparation of a multilayer coating and to suitable aqueous, non-yellowing coating materials or coating compositions suitable for its implementation. These can be used as water-dilutable plain base coats or metallic base coats and are usually oversprayed with a clear lacquer, which is water dilutable or dissolved in organic solvents. The vehicles employed can be used as industrial paints, especially, for example, in the automobile industry. They lead to coatings which, aside from a good optical effect and excellent mechanical properties, result in good intermediate adhesion, show little swelling in water and can be applied trouble-free by electrostatic spraying. In addition, they are suitable for repair purposes, since they lead to excellent properties already when cured at low temperatures, such as 80.degree. C.BACKGROUND INFORMATION AND PRIOR ARTIt is well known that substrates may be provided with several superimposed coating layers in or...

AI Technical Summary

Benefits of technology

Due to the use of vehicles that have not been cross linked, a good flow, which leads to smooth surfaces, is achieved with the inventive coating materials. Moreover, the bronze orientation in metallic lacquers is improved. The water resistance of the coatings achieved is excellent. When used in the motor vehicle sector, excellent stone chipping resistance is achieved.

The inventive, water-containing and water-dilutable coatings have the advantage that they do not yellow. They can be used as base lacquer and as covering lacquer. Their use as base lacquer is preferred. After a predrying, these base lacquers can be painted over with a clear covering lacquer and both layers can then be dried (stoved) together. As clear covering lacquers, the known, conventional, solv

Problems solved by technology

The problem therefore existed of converting such a lacquer system to a water-dilutable basis.

In the development of such water-dilutable systems, problems arise, which are difficult to solve and are attributable to the special properties of water and to the low heating temperatures, at which practically no cross linking reactions take place.

However, it was not possible to combine all the desired properties optimally in one vehicle.

This conversion process is very costly and also very susceptible to failure, because the effectiveness of the dispersion stabilizer can be affected by variations in the synthesis process.

At low stoving temperatures, this resin acts like a plasticizer and worsens the moisture susceptibility of the film.

Because of the non-volatile salt content in the film, adhesion problems arise in multilayer lacquer structure.

In so doing, films are formed in multilayer structures, which are not sufficiently resistant to the effects of moisture.

Here also, special selection conditions cannot be inferred for the preparation of metallic base coats.

The film properties show a high solvent sensitivity, which leads to difficulties during t

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